1. Field of the Invention
The present invention relates to magnetic disc and, more particularly, to a flexible magnetic disc for use in a system and method which includes a regulator plate and performs magnetic recording or playback by protruding a magnetic head beyond a regulation level of a rotatable sheet of magnetic recording medium.
2. Description of the Prior Art
Generally, a flexible magnetic disc of the type described has a thin disc sheet which is encased in a jacket. During a recording or playback mode of operation, only the thin disc sheet in the jacket is rotated while a magnetic head, or transducer, is held in contact with the disc sheet through an opening formed in the jacket. So long as the disc has a relatively large diameter, it is prevented from swaying or warping when the head is protruded or thrust toward it due to a centrifugal force developing in the disc, allowing the head to stably abut against the disc.
Concerning a disc having a relatively smaller diameter, however, the centrifugal force is smaller than one acting on the larger diameter disc so that protruding or projecting the head toward the disc causes a recording surface of the disc to warp away from the head. Such instability of contact of the head with the disc is brought about in the case where the disc accommodated in a jacket has a small diameter such as about 5 centimeters (cm). Needless to say, accuracy of abutment or contact of the head against the recording surface of the disc is a primary requisite for stable recording or playback of signals. The abovementioned warping, or swaying, of the head degrades the accuracy of abutment and, thereby, prevents signals from being recorded or reproduced with stability.
In light of this, there has been proposed a construction which regulates the warping of a disc sheet away from a head, or toward the back of the disc sheet where the head is absent, when the head is thrust into contact with the disc sheet, as disclosed in Japanese Patent Laid-Open Publication Nos. 113459/1982 and 113460/1982, for example. The regulation is implemented by a regulator plate or board which has a recess so shaped as to regulate or limit warping of the sheet and presses against that part of the back of the sheet which corresponds in position to the head.
However, we have found that the mechanism relying simply on the regulator plate is not always successful to provide desirable contact of a head with the recording surface of a disc sheet.
One of the causes of such unstable head-disc contact is the physical curling of a magnetic disc itself.
The situation concerned with curling will be discussed in relation with a prior art flexible magnetic disc. As shown in FIG. 1, a flexible magnetic disc, generally 10, comprises a jacket 12 in which a magnetic recording sheet 14 is housed. In an exemplary configuration, the jacket 12 may be dimensioned about 5 cm at each side while the sheet 14 may have a diameter of about 5 cm and a thickness of about 40 microns (.mu.m). A hub, or core, 16 is rigidly mounted on a radially central area of the sheet 14 so that it may be chucked by a rotary drive shaft through a central opening of the jacket 14 in order to impart rotation to the sheet 14. The jacket 12 has an opening 18 which allows the regulator plate and the magnetic head to abut respectively against the opposite surfaces of the sheet 14.
The sheet 14 has such a configuration as one shown in FIG. 2. FIG. 3 shows the sheet 14 in a section along line III of FIG. 2 with the core 16 removed, representing a curled condition of the sheet 14. In FIG. 3, the degree of curling of the sheet 14 is indicated in terms of the height h of the circumferential edge of the sheet 14 measured from a reference plane or level 20. The curl h may be measured by, for example, laying a magnetic sheet gently on a flat plate under a room temperature and humidity condition.
Usually, an auxiliary or additional layer referred to as a "balance coating" is deposited on that surface of a base material, or support member. The balance coating is opposite to the recording surface in order to cope with curling. The auxiliary layer is ordinarily made of a material which is substantially common to the material constituting a recording material layer. Nevertheless, curling is unavoidable due to the influence of biaxial stretching which is resultant from the production of the support plate of the sheet and the influence of subtle changes in ambient conditions in the course of production. Further, curling may occur while a core is shaped on the sheet.
Even though a curl at the production stage of the sheet may be eliminated, it may be brought about by aging of the sheet. Additionally, where such a magnetic disc is used with an electronic still camera, it is quite difficult to free the disc from curling under, for example, ordinary conditions of use of a camera, that is, a hot and humid condition to a cold and dry condition. Curling is, therefore, physically inevitable although to one angular degree or another.
For example, the curl h was actually measured to be as large as about 0.3 millimeter (mm) at the minimum and about 1.5-3 mm at the maximum even with a magnetic sheet having a balance coating thereon.
Where a magnetic sheet with such a curl is used for recording or playback purpose, rotating the sheet at, for example, 3,600 rpm is successful to somewhat straighten the curl with the aid of the centrifugal force but not sufficiently. A prior art recording and/or playback apparatus using a magnetic sheet is shown in FIG. 4 which employs a regulator plate 32 as a solution to the above-mentioned curl problem. In FIG. 4, the regulator plate 32 is semi-permanently fixed in place adjacent to the back 24 of the magnetic sheet 14 in positional correspondence with a magnetic head 30 which neighbors a recording surface 22 of the sheet 14. When the head 30 urges, or is protruded against, the sheet 14, the regulator plate 32 serves to limit the warp of the sheet 14.
An example of the regulator plate 32 is shown in a perspective view in FIG. 5 and in a section along line VI of FIG. 5 in FIG. 6. As shown, the regulator plate 32 has a flat guide surface 38 for guiding the back 24 of the sheet 14. The guide surface 38 is formed with a recess 36 in an intermediate portion thereof which is open at one end thereof.
As shown in FIGS. 7 and 8 which are respectively sections along lines VII and VIII of FIG. 4, the regulator plate 32 cooperates with the head 30 which is thrust toward the plate 32 in regulating the position of the sheet 14. Despite such regulation, however, a prior art magnetic disc cannot be straightened enough to meet the relative positional relationship requiring an accuracy on the order of, for example, 20 .mu.m.
In practice, even if the abutting position of the sheet is limited by the urging force exerted by the coactive regulator and head, signals recorded on or reproduced from the sheet by the head develop high-frequency fluctuation of the envelope as exemplified by FIG. 9A and low-frequency fluctuation of the envelope as exemplified by FIG. 9B.
In FIGS. 9A and 9B, a video signal 50 is shown which was produced by frequency modulating (FM) a color difference signal and a luminance signal and reproduced from a magnetic disc, with respect to one rotation of the disc. FIG. 9A suggests that even the regulation relying on the coactive regulator 32 and head 30 cannot eliminate small oscillations of that surface of the disc which is engaged by the head, the oscillations being reflected particularly by the high-frequency fluctuation of the envelope 52 of the FM signal 50. Concerning the waveform shown in FIG. 9B, curling of the sheet 14 itself is reflected; if the magnetic material has orientation, its influence will be combined with the influence of curling.
We recorded and reproduced signals into and from a magnetic sheet with a large curl by using a regulator plate and a magnetic head, and found the recording surface of the magnetic sheet oscillating minutely. Such oscillation appears itself in the signal waveform as high-frequency noise.
Other factors which effect the stability of head contact as critically as the above-described curling of a magnetic disc are the mechanical accuracy of the recording and/or playback mechanism, the accuracy of the core of the disc, and whether the disc is properly chucked.
For example, even though the accuracy of a hold reference surface of a recording medium is lower than the order of 20 .mu.m at the core of a magnetic disc, a record reference surface of the recording medium in rotation is caused to move up and down, and this effects the head contact even in the part of the disc which is regulated by the regulator plate.
An attempt may be made to set up good head contact free from the influence of curling, mechanical accuracy of the disc core, that of the recording/playback mechanism, and chucking by thrusting the head a larger amount toward the regulator plate. Such an attempt, however, increases the pressure exerted by the head on the recording surface of the magnetic sheet and, thereby, enhances the wear of the head and the disc to noticeably shorten their lives. This wear problem is especially critical when it comes to a still picture system which reproduces a still picture from a video signal recorded on a magnetic disc, inasmuch as a playback head of the system repeatedly traces a single track when the system is in a playback mode. Thus, recording and playback operations associated with this type of flexible magnetic discs have to satisfy two dilemmatic requirements:
(1) The amount of protrusion of the head has to be increased in order to achieve stable head contact; and
(2) Taking into account the lives of the head and magnetic sheet as well as the need for repetitive playback, the contact pressure exerted by the head on the magnetic sheet has to be decreased.